Process for production of aromatic polycarboxylic acids



United States Patent O PROCESS FOR PRODUCTION OF AROMATIC POLYCARBOXYLIC ACIDS Ewald Katzschmann, Witten (Ruhr), Germany, assignor, by mesne assignments, to Imhausen Werke, G. m. in. BE, Witten (Ruhr), Germany, a corporation of Germany No Drawing. Application July 31, 1952, Serial No. 301,996

Claims priority, application Germany August 6, 1951 9 Claims. (Cl. 260-524) This invention relates to the production of aromatic carboxylic acids from alkylaromatic compounds. More particularly, it relates to the production of the various isomeric phthalic acids from the corresponding xylene isomers. It relates especially to the production of terephthalic acid from p-xylene.

In the application, Serial No. 280,387, filed April 3, 1952, of Ewald Katzschmann, now abandoned, there is described a highly advantageous process for converting p-xylene practically quantitatively into the terephthalic acid or its esters at elevated temperatures solely with the aid of oxygen or oxygen-containing gases. In this process the terephthalic acid-containing p-toluic acid obtained as an intermediate product is esterified. The terephthalic acidester, if desired, is removed, and the p-toluic acid ester is oxidized with oxygen or oxygen-containing gases. Surprisingly, it has been found that in the oxidation of p-toluic acid esters with oxygen practically only the nuclear methyl group is attacked while the alcohol radical combined with the carboxyl group of the p-toluic acid remains unattached.

More particularly, this application discloses that the oxidation of p-toluic acid esters with oxygen or oxygencontaining gases at elevated temperatures in liquid phase, under pressure if desired, and advantageously in the presence of oxidation catalysts, proceeds at a rather fast rate and gives excellent yields of pure terephthalic acid monoesters. Moreover, under conditions of oxidation practically no ester interchange of the terephthalic acid monoester occurs so that the latter is obtained in practically pure form.

The aforesaid application also discloses that the inventive idea thereof is capable of generalization. Thus, in the oxidation of alkylaromatic compounds other than the xylenes, the rate of oxidation to the corresponding aromatic polycarboxylic acid is in no way uniform. In the oxidation of other alkylaromatic compounds intermediate carboxylic acids are formed which, like p-toluic acid in the case of p-xylene, are much harder to oxidize further than the starting material or other intermediates, so that the oxidation to polycarboxylic acids undergoes a marked retardation. As in the oxidation of p-xylene, the rate of oxidation of other alkylaromatic compounds to the corresponding polycarboxylic acids also may be greatly increased if the intermediate carboxylic acids formed are converted into esters, and these are further oxidized.

It has now been found that in the case of p-xylene or any other alkylaromatic compound, the rate of oxidation to the corresponding polycarboxylic acids also may be greatly increased if the intermediate carboxylic acids formed, which are relatively more difiicultly oxidizable than the starting material or other intermediates, are converted into anhydrides, and these are further oxidized. Thus, ditficultly further oxidizable carboxylic acids occurring as intermediates, which, therefore, contain besides one or more carboxyl groups also alkyl radicals and/or 2,764,61 l P tented Sept. 25, 1956 partially oxidized alkyl radicals, are converted to anhydrides in the known manner and the resulting anhydrides are further oxidized. In the case of p-toluic acid obtained in the oxidation of p-xylene, for example, it is converted to p-toluic anhydride by reaction with acetic anhydride and the anhydride further oxidized.

The fundamental idea of the present invention-facilitating the oxidation of alkylaromatic compounds to the corresponding polycarboxylic acids by the conversion of ditficultly oxidizable intermediates to more readily oxidizable anhydrides-can be applied not only to the various methylbenzenes, such as xylenes, mesitylene, pseudo cumene, durene, etc., but also to alkylaromatic compounds with longer alkyl chains, such as methylethylbenzenes, cymenes, etc., and also to alkylated polynuclear aromatic compounds, such as naphthalene, diphenyl, anthracene, etc.

The conversion of relatively diificultly oxidizable intermediate products of oxidation into more readily oxidizable anhydrides may, if desired, also be performed repeatedly; thus, alkylmonocarboxylic acids formed on oxidation of polyalkylaromatic compounds may be converted to anhydrides and these products oxidized to alkyldicarboxylic acids, followed by conversion to anhydrides, and the latter further oxidized to aromatic tricarboxylic acids, which may still contain alkyl groups.

The conversion of the sparingly oxidizable intermediate carboxylic acids to more easily oxidizable anhydrides is preferably carried out before their concentration in the reaction mixture has increased to such an extent that the rate of oxygen absorption has appreciably decreased. It is fundamentally without significance whether the conversion of the diflicultly oxidizable intermediate carboxylic acids to the more easily oxidizable anhydrides takes place after their complete or partial separation from the other components of the reaction mixture, or in their presence; the most suitable method in each case can be determined by a simple test.

It is especially advisable to remove the diflicultly oxidizable intermediate carboxylic acids continuously, or at short intervals, from the reaction mixture or from a part thereof, or to convert them therein into more readily oxidizable anhydrides, to make up the consumed starting material and, if necessary, to remove the products of oxidation in such a way that the composition of the reaction mixture and the rate of reaction will remain substantially constant.

The oxidation steps of the process of this invention are carried out under essentially the same conditions as discussed in the aforesaid application, Serial No. 280,387.

By the method according to the invention, polyalkylaromatic compounds can be readily converted into the corresponding polycarboxylic acids.

The following examples are given in illustration of the invention.

Example 1 m-Toluic acid is obtained in the known manner by oxidizing m-xylene with air in the presence of cobalt salts of C4C9 fatty acids (the forerun acids obtained in the oxidation of parafiin).

1000 g. of m-toluic anhydride is prepared in the known manner by reaction of m-toluic acid with acetic anhydride. The rn-toluic anhydride is then oxidized in the presence of 2 g. of the aforementioned cobalt catalyst at C. with 2 liters of air per minute for 24 hours. The 1035 g. of oxidate obtained is saponified with KOH solution and the acids set free by acidification. After drying, the mixture of acids obtained is extracted in the warm with 6 liters of benzene. 98 g. ofisophthalic acid (M. P. 344 C., acid No. 669) remains undissolved.

If 1000 g. of free m-toluic acid is treated with air for 48 hours under identical conditions, only 12 g. of isophthalic acid is obtained upon processing as described.

Example 2 p-Toluic acid is obtained in the known manner by oxidizing p-xylene with air in the presence of cobalt salts of C4-C9 fatty acids (the forerun acids obtained in the oxidation of pa'rafiin).

1000 g. of p-toluic anhydride is prepared in the known manner from p-toluic acid by reaction with acetic anhydride. The p-toluic anhydride is then oxidized in the presence of 2 g. of the aforesaid cobalt salts with 1.5 liters of air per minute at 125 C. for 42 hours, whereupon 1075 g. of oxidate is obtained. The oxidate is refluxed with 1500 g. of methanol, with the addition of 3% H2504, for 45 hours. From the methanol solution obtained, 95 g. of dimethyl terephthalate separates. The ester is practically pure after one recrystallization from methanol according to the properties: Acid number 1.2; saponification number 573; melting point, 140 C.

p-Toluic acid itself cannot be oxidized under these conditions because of its melting point, 180 C.

What I claim and desire to protect by Letters Patent is:

1. The process for the preparation of an aromatic polycarboxylic acid which comprises oxidizing the anhydride of a difiicultly oxidizable alkyl-aromatic monocarboxylic acid in liquid phase with an oxygen-containing gas at an elevated temperature.

2. The process of claim 1, wherein the oxidation step is carried out in the presence of an oxidation catalyst.

3. The process of claim 1, wherein the oxidation step is carried out under superatmospheric pressure.

4. The process of claim 2, wherein the aromatic polycarboxylic acid is removed continuously from the oxidation vessel and the consumed anhydride of the difficultly oxidizable monocarboxylic acid is made up continuously so as to maintain the composition of the reaction mixture substantially constant.

5. The process for the manufacture of a phthalic acid which comprises oxidizing the anhydride of a toluic acid in liquid phase with an oxygen-containing gas at an elevated temperature in the presence of an oxidation catalyst.

6. The process of claim 5, wherein a cobalt salt is employed as the oxidation catalyst.

7. The process of claim 6, wherein o-toluic acid anhydride is the starting material.

8. The process of claim 6, wherein m-toluic acid anhydride is the starting material.

9. The process of claim 6, wherein p-toluic acid anhydride is the starting material.

References Cited in the file of this patent UNITED STATES PATENTS 1,815,985 Pansegrau July 28, 1931 2,479,067 Gresham Aug. 16, 1949 2,531,172 Toland Nov. 21, 1950 2,559,147 Emerson et al July 3, 1951 FOREIGN PATENTS 623,836 Great Britain May 24, 1949 OTHER REFERENCES Denham, Beilstein (Handbuch, 4th ed.), vol. 9, pp.- 464, 476, 484 (1926).

Dunbar, Chem. Abstracts, vol. 44, col. 5313 (1950). 

1. THE PROCESS FOR THE PREPARATION OF AN AROMATIC POLYCARBOXYLIC ACID WHICH COMPRISES OXIDIZING THE ANHYDRIDE OF A DIFFICULTLY OXIDIZABLE ALKYL-AROMATIC MONOCARBOXYLIC ACID IN LIQUID PHASE WITH AN OXYGEN-CONTAINING GAS AT AN ELEVATED TEMPERATURE. 